There has been a long-felt need for non-destructive inspection methods that can be used to determine the integrity of a bond between two items. This need has been felt particularly in the case of bonds formed at the micro-level. The micro-level can be defined as involving at least one material with a dimension on the order of 0.005 inch or smaller. Such inspection methods might be applied to testing ball and wedge bonds, thin coatings, circuit traces, ribbon bonds, solder balls, surface mount components, PIN grid arrays, and MIMMs commonly used in microelectronics interconnects. The materials joined in these applications include, but are not limited to, silicon, silicon carbide, aluminum, gold, gallium arsenide and the like.
For example, ball bonds used in connecting silicon wafers to external circuits through very fine wires are typically tested, if at all, according to military specifications which require a pull test of each bond. This test is performed using a machine which sequentially hooks each wire and applies a predetermined pulling force to determine whether the associated bond will hold. This technique has significant limitations. In particular, the inventor has discovered that if this test is performed repeatedly on the wires of the same device, an increasing number of wires typically pull loose with each succeeding test. This result implies that the test does not truly qualify as non-destructive. That is, each application of a pulling force to a wire weakens its bond and repeated testing will actually break the bonds. It is possible that a bond might pass a single pull test of this type, but that the test would leave the bond precariously connected and destined for failure in the field when subjected to ambient vibration, shock, or temperature variations.
In the case of bonds having larger dimensions, such as pipe seams, welds used in automotive and marine manufacturing, etc. various x-ray and acoustic techniques have been applied to analyze the condition of an interface between two items. Laser ultrasound techniques have also been proposed. For example, U.S. Pat. Nos. 4,659,224 and 4,966,459 to Monchalin, U.S. Pat. No. 5,081,491, to Monchalin et al., and U.S. Pat. No. 5,137,361 to Heon et al. disclose the results of early research in this field. U.S. Pat. No. 5,103,676 to Garcia et al. shows a further method of laser ultrasonic process monitoring.
Laser techniques have also been considered for use with smaller bonds such as those found in semiconductor circuits. U.S. Pat. No. 5,201,841 to Lebeau et al. proposes a thermal gradient technique, and Japanese Patent Publication 62-7198 (Jan. 14, 1987) by Hitachi Research Corp. appears to propose a laser technique.
The inventor's prior U.S. Pat. Nos. 5,420,689, 5,424,838, and 5,302,836 disclose lighting methods and apparatus useful in small-scale laser ultrasonic measurement. U.S. Pat. No. 5,535,006 to Siu et al. builds on the inventor's earlier work and discloses a method of evaluating integrity of adherence of a conductor bond to a substrate.
However, as far as the inventor is aware, none of these prior systems has provided an effective alternative to pull testing of wire bonds, or an effective method for analyzing thin film coating integrity at the micro-level. The results produced by the systems disclosed in the inventor's own prior patents, while encouraging, were not consistent enough for industrial application.
Thus, there is a need in industry for improved methods and systems of this type that will provide repeatable, accurate, and truly non-destructive testing capability.